Power and free conveyors are well known systems made up of a power track and a free track and a pair of trolleys capable of traveling along the free track and supporting a carrier. Each leading trolley in a power and free system includes a driving dog portion which extends toward the power track and which is engageable by a pusher dog carried by a moving chain on the power track. When the pusher dog and the driving dog are engaged, the trolley is pushed along the free track by the moving power chain. When the driving dog is retracted, or otherwise disengaged from the pusher dog, the trolley stops moving.
Power and free conveyors often include accumulation tracks or sections where a number of carriers can be stored in a closely spaced or contacting relationship out of the way of the main portion of the conveyor system until they are needed. In order to accumulate carriers, it is necessary to disengage the driving dogs of the trolleys supporting the carriers from the pusher dog. This is usually accomplished by having the accumulation section of the track at a different elevation than the main portion of the track so that a carrier directed into the accumulation portion will roll down and away from the pusher until it runs into a stationary object The slope must be steep enough to ensure that the carrier moves quickly away from the pusher to avoid jams and to impart a sufficiently high speed to the carrier to ensure that it continues to roll along the accumulation section of track until it contacts a stop or another carrier. Additional carriers can be accumulated in this manner until the accumulation section of track is full. However, because carriers are periodically removed from the downstream end of the accumulation section, new carriers can be continuously accumulated.
Trolleys and carriers are made of steel or similar alloys and consequently are quite heavy. Each pair of trolleys supports a carrier which is also very massive and which may be supporting a heavy workpiece at any given time. Such carriers possess large kinetic energies when they roll along the accumulation section of the conveyor track, and this energy must be dissipated in order to stop the carrier. Because the carriers and trolleys are made from metal, a significant amount of energy from each collision between carriers is dissipated as sound waves, and each impact produces a relatively loud noise. Since a given conveyor line may have multiple accumulation portions, and a plant may have multiple conveyor lines each equipped with multiple accumulators, the noise produced by the accumulation of carriers is substantial and frequent. This noise can be unpleasant to anyone in a plant where accumulators are used, and, to employees required to work in close proximity to these accumulators, the noise can have adverse health affects as well.
The high frequencies generated by this metal-to-metal contact can be particularly damaging to hearing. In addition, the trolleys are often not accumulated at regular intervals. Thus, the sound of each successive impact may come as something of a surprise to nearby employees. An employee who is startled by such sudden noises may have difficulty concentrating on his job. Attempts have been made, therefore, to reduce the noise created by these accumulations. The normal solution is to coat the metal surfaces normally making contact with rubber or a similar material which will absorb the impact of the collisions. While this can reduce the noise problem to some extent, it also increases the spacing between adjacent carriers in an accumulation line, especially when enough cushioning material is used to reduce the noise level by a noticeable amount. This increased spacing changes the location of each trolley in the accumulation line and makes it difficult for a system controller to keep track of the carriers. In addition, this altered spacing may prevent the carriers from aligning properly with other elements of the conveyor system, such as unloading platforms along the accumulation track. In addition, adding these materials to existing systems usually requires that the trolleys be modified to accommodate the cushioning. This is often cost prohibitive. It would therefore be desirable to reduce the noise generated by the accumulation of carriers in a conveyor line without adversely affecting the spacing of the carriers, and which noise reduction could be employed on existing systems without extensively modifying the carriers.